The “Bergantino” nymphaeum, called “Diana’s Baths”, was asked for by the emperor Domitian, inspired by that of Tiberius in Sperlonga. A natural cave (perhaps a pozzolan quarry) was adapted with tanks and structured environments comprised of walls of mixed materials. The side walls were covered with huge slabs of marble and on the circular basin floor was a polychrome mosaic with marine scenes. Everything was embellished with sculptures: Polyphemus, Scylla, Centauri.
This monument also had a spa facility, first built by Domitian and then by Pope Alexander VII Chigi. Here were also the brought the vessels used in Emperor’s naval battles or by the Pontiff.
The most important artifact, however, was the Emissary, a pipeline built in the fourth century BC to regulate the water level. The pipeline ends after a long underground journey, in a location “The Mole” at Castel Gandolfo where there were ancient water mills.
The story of this Emissary is very unique. The Delphic oracle had predicted the victory of the Romans over the Veii, where the waters of the lake had arrived at the sea. Thus, around 398 A, the work was realized to create this long mammoth tunnel nearly 1800 m, 1.80 m high and excavated 128 m below the crater of the Lazio volcano. The Emmisary has 5 ventilation shafts.